Conventional optical transport networks, including Synchronous Digital Hierarchy (SDH) used outside North America and Synchronous Optical Network (SONET) used in North America, are all service providing means based on manual operation, and the protection methods are all fixed protection method based on ring network or line. Here the service refers to network connection, and thus the optical network service means optical network connection. With the rapid development of data services represented by the Internet, the requirement on transmission bandwidth expands constantly, and the operating mode in which the service is provided manually and the ring network protection method become more and more inadaptable to the requirement of the service.
In this context, the Internet Engineering Task Force (IETF) has extended the control plane protocol originally used for data exchange in a packet switched network, Multiple Protocol Label Switching, so as to be used in an optical network, which is known as Generalized Multiple Protocol Label Switching (GMPLS). The GMPLS is a set of protocols based on the IP technologies, including automatic discovery, routing and signaling protocols, which as the control plane basis of the optical network, supports automatic providing of optical connection and network failure restoration.
The GMPLS introduces a new Link Management Protocol (LMP) to support link automatic discovery; the connection relationship of the adjacent network elements can be obtained through the link automatic discovery, thereupon the information can be issued to other network elements in the network through a routing protocol. In GMPLS, link state advertisement is implemented through extending “Open Shortest Path First—Traffic Engineering (OSPF-TE)”; then an end-to-end supporting network connection is established through extending “Resource Reservation Protocol-Traffic Engineering (RSVP-TE)” or “Constraint based Routing Label Distribution Protocol (CR-LDP)”.
The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) assigns the standardization work of the control plane of optical network to its Study Group 15 (SG15), which unifies the standardization work of the control plane of optical network as “Automatically Switched Optical Network (ASON)”. ASON focuses mainly on making standards in terms of the requirement, framework and interface of the control plane of optical network, and a standard system with the ITU-T recommendation G.8080 as the core was gradually formed. It needs to be pointed out that the ASON standard itself does not include implementation of a protocol layer. ASON adopts a protocol of other standardization organizations, for example, the GMPLS protocol as the protocol basis.
Conventional ring network protection means, for example, a shared Multiplex Section Protection Ring (MSPRing) can provide a service restoration time of 50 ms, but need to reserve 50% of the bandwidth for protection, resulting in a low utilization ratio of bandwidth. A more serious problem with the ring network is the limitation introduced by the ring network, since it is required that the capacities of the links contained in the shared Multiplex Section Protection Ring must be consistent with each other, if the capacity of at least one link on the ring is used up, it should be implemented a capacity-expansion processing. There are two methods to implement capacity-expansion: (1) upgrading the capacity of the ring (for example, upgrading a ring from STM-16 to STM-64), during the process of which, the current services on the ring need to be rerouted; (2) establishing another ring and bearing the services that cannot be accommodated by the original multiplex section ring onto the new ring, but such a method in which parallel rings are employed brings about difficulties in maintenance and management. In summary, a ring network suffers from problems of such as low utilization ratio of bandwidth and inconvenient capacity-expansion, which can not meet the requirement on transporting data services with fast-varying traffic.
In an ASON, by introducing a control plane, the traffic affected by a network failure can be restored dynamically through rerouting; through MESH networking, the ASON can support restoration of multiple failures and provide higher service reliability. For a MESH, the planning thereof is directly driven by service and the corresponding link bandwidth can be planned according to end-to-end traffic, which is more convenient and flexible compared with a ring network. Combining with wave division technology (DWDM), the ASON can dynamically change network topology in response to the variation in traffic demand and meet the requirement of data service very well. The ASON based on a GMPLS control plane has the above-mentioned advantage of flexibility, but at present, the problem is in that the restoration time for implementing rerouting according to Resource Reservation Protocol-Traffic Engineering (RSVP-TE) is on the order of second, which can not meet the requirements of the operators, saying nothing of the requirement of voice service for a protection time of 50 ms. The problem of long restoration time of an existing MESH hinders the operators from adopting the ASON based on MESH networking.
To solve the problems in restoration reliability and restoration speed of a MESH, the “RSVP-TE Extensions in support of End-to-End GMPLS-based Recovery” draft drawn by the Common Control and Measurement Plane (CCAMP) workgroup of the IETF presents a mechanism of “shared-mesh-restoration”. The basic idea is to find a restoration path uncorrelated to the failure on the working path for connection while establishing the working path. The working path operates signaling, reserves resources, establishes cross connections and provides end-to-end service transport ability; while the restoration path runs signaling, reserves resources but does not establish cross connections, thus the resources on the restoration path can be shared and used for protecting multiple failure-uncorrelated working paths. When a working path fails, the establishment of a cross connection of the corresponding restoration path is triggered through signaling. Specifically, this process can be divided into two main steps: the first step is to implement link resource reservation in the network elements on the restoration path, and the second step is to drive the network elements on the restoration path to establish a cross connection to activate the restoration path after the working path fails. The first step is accomplished before a failure occurs, without a real-time requirement; while the interrupt time of the service is affected by the accomplishing speed of the second step, which thus has a real-time requirement. The two steps of all the existing MESH restoration solutions are accomplished by driving through a massage-based protocol (e.g., the GMPLS extended RSVP-TE protocol) of the control plane, instead of by a bit-based protocol similar to that for Multiplex Section Ring Protection. It needs more complicated software to support the message-based protocol, and needs coordination among multiple tasks of the operating system to implement the operations based on a protocol; compared with the bit-based protocol, the operations of the message-based protocol takes more time, and the jitter of the accomplishing time is greater, whereby the real-time nature in establishing the restoration path in the second step is affected. Therefore, although the time for establishing a restoration path with reserved resources is shorter than that for the dynamical restoration without reserved resources, it is still very difficult to achieve a restoration time of 50-200 ms through signaling transported via IP; especially in the case of a large number of path restorations due to a failure on a high-capacity link, it is difficult to ensure the restoration time.